1. Field of the Invention
The present invention pertains to the field of electron beam tubes and more particularly to an ungridded electron gun having an electrically isolated focusing electrode and a modulating anode.
2. Description of Related Art
In travelling wave tubes (TWTs) used in microwave power modules, it is desirable to be able to selectively shut off the electron beam current or at least reduce its magnitude to a tolerably low level. This can be accomplished by switching the focusing electrode of the electron gun to a voltage potential that is negative with respect to the cathode. Alternatively, the cathode voltage potential can be switched towards ground in order to establish a negative bias on the cathode with respect to the focusing electrode. In a typical modulating anode electron gun, the anode is generally switched from essentially ground potential to a potential approximately equal to the cathode potential, thereby reducing the electron beam current effectively to zero.
However, many electron guns are designed to exhibit a high perveance, which is defined as the ratio of the space-charge-limited beam current to the gun cathode-to-anode voltage raised to the three halves power. A higher perveance thus indicates that the emitted electron beam is more heavily influenced by space-charge effects. In such a system, the voltage that must be applied to the focusing electrode in order to completely cut off the beam current becomes unacceptably large. For example, FIG. 1 is a plot of a normalized focusing electrode cutoff voltage as a function of anode microperveance. The open circles, e.g., 110, are measured cutoff voltage ratios of various electron guns having different microperveance values. The microperveance of a given electron gun is a function of its geometry. It can be observed from FIG. 1 that the relationship of the cutoff voltage ratio to the microperveance is quite linear. This linear relationship is illustrated by curve 102, which is an empirical fit to the measured cutoff ratios of several different electron gun designs. A linear fit 106 to the data, shows that the normalized cutoff voltage is related to microperveance by a ratio of approximately 0.44. Thus, as the anode microperveance approaches a value of 2.1, the voltage that must be applied to the focusing electrode approaches the level of the cathode-to-anode voltage itself. Switching a voltage of this magnitude, which may be several thousand volts, poses a difficult challenge because modern solid-state voltage switches cannot easily handle voltage magnitudes greater than approximately 2000 volts. Thus, it would be desirable to provide a system for switching voltages within a high-perveance electron gun to achieve full beam cutoff while overcoming the difficulties of switching high-magnitude voltages described above.